4-2 THE VOYAGE OF H.M.S. CHALLENGER. 



the " otic process;" the former (pd.) is a mere spike always miossified at its tip, and touch- 

 ing the top of the epiptqrygoid with which it was continuous. 



The latter (ot.p.) forms an almost circular dome-like vestibule to the " middle ear," 

 and this part is largely unossified as yet. The main stem of the bone, however, is seen 

 on its inner face to be solid and phalangiform, and largely ossified, the condyloid lower 

 articular facet, and the convex j^art above, which articulates with the ear-capsule, are 

 still, and indeed, permanently, soft; to the rim of this hollow " porch of the ear," the very 

 Batrachian cartilaginous "annulus tympanicus" is attached, and in its notch the orbicular 

 " extrastapedial" fits (PI. XII. fig. 1, a.ti/.,e.st.). This latter part is, as I have already shown, 

 the distal end of the " epi-hyal " element, and the proximal end is the " mediostapedial," 

 or stem of the " columella," whose base is the true periotic stapes (st.). The axis of each 

 mandible {mk) is still confluent with its fellow of the opposite side, and is wholly 

 unossified. The shaft of the columella (go.), and the shaft of the cerato-hyal (PI. XL 

 fig. 7, c.hy.) are both more extended than in the last, and the hypo-hyal is shorter and 

 more bulbous ; the " basi-hyo-branchial " (PL XL fig. 7, b.hy.,h.br.) gives off an unossified 

 glosso-hyal spike in front, and a pair of hypo-branchials {h.br.) are articulated to its pro- 

 jecting lobes behind. 



Transversely Vertical Sections of the Head. 



A description of these slices will complete what I have to say of this ripe stage. 



First Section. — This section (PL XII. fig. 4) is made through the fore margin of the 

 thick premaxillaries (px.) and the external nostrils (e.n.) The alinasal tubes (al.n.) are 

 cut through, which are, essentially the same as the separate narial valves of the Frog, 

 formed out of the external segment of the large upper labials. The narrowness of the 

 face downwards is here shown, and the flatness of the face above. 



Second Section. — The next section (PL XII. fig. 5) takes in the fore part of the nasal 

 capsule {n.io.) as well as the alinasal valve {al.n.), and on this front part the prenasal {p.n.) is 

 shown, growing almost directly downwards. This extension of the skull base serves here 

 as a model for the premaxillaries, as in the Birds, but in these the prenasal soon loses 

 this downward direction, and grows forwards as in Selachians, in both cases the " pro- 

 rhinals " are either suppressed, or but slightly developed, as a rule. 



Third Section. — Here (fig. 6) there are four passages {e.n.,n.c.), for the nasal pouches 

 lie beneath the alinasal outlets in front. Above, the alinasal has passed into the aliseptal 

 lamina {cd.sp.); below, the nasal floor («/) is seen to bind upon the hinder part of the 

 prenasal spike {p.n.). The nasal septum {s.n.) does not reach the base at this part, nor is 

 the beginning of the aliseptal lamina continuous with the nasal wall and floor («/)• The 

 latter cartilage is now protected, externally, by the fore edge of the maxillary {mx.); and 

 the hollow of the palate has begun, as shown in the space between the premaxillaries (jja?.). 



